By introducing the use of wavelength division multiplexing (WDM) in optical fiber communication networks the bandwidth of and thereby the transmitted information amount in such networks can be made much higher than before, without using extremely high transmission rates. The information is instead transmitted on a number of parallel channels which each one comprises a definite, separate wavelength region or wavelength band. Systems are is presently introduced comprising 4-16 channels having a transmission rate or bit rate of 2.5 Gbits/s per channel. As regarded in a longer perspective of time, certainly still more channels will be used. Thus, it is completely realistic to use a number of 16-32 channels and in laboratory situations functioning transmission systems using 128 channels have been demonstrated. Further, in the same way, certainly also the bit rate per channel will be significantly increased, for example up to 10 Gbits/s. Still higher transmission rates have been used in laboratory situations such as bit rates of 20, 30 and 40 Gbits/s and they will perhaps also be used in the future.
For each channel and wavelength region in wavelength multiplexed transmission a separate light source such as a suitable semi-conductor laser must be used, the light issued by the laser in addition having to be capable of being modulated in order to obtain a bit stream carrying useful information. However, one of the main problems is to achieve such laser transmitters, since they must have a narrow optical line width, i.e. have a small chirp. It can be accomplished by among other methods introducing external modulation, i.e. that the laser is driven by a constant current, and by making the modulation by means of a separate intensity modulator or an intensity modulator monolithically integrated with the laser, for example of electroabsorption type. The laser should be either type DFB, i.e. a laser having distributed feedback, or of type DBR, i.e. a laser having a distributed Bragg reflector, in order to ensure that when operating the laser only one longitudinal electromagnetic mode lases.
The wavelength region which presently is most interesting for wavelength multiplexing comprises the range of about 1530-1560 nm. This is the range for which good fiber amplifiers are available, such as erbium doped fiber amplifiers (EDFA:s). In the future other wavelength ranges can start to be used such as for example about 1300 nm.
Typically, presently used light emitting and modulating devices are constructed so that laser transmitters of e.g. type DFB are manufactured for different wavelengths, at which the respective laser transmitter can be activated for emitting light. The lasing wavelength of such a DFB laser is determined by the active refractive index in the active layer of the laser and of the pitch ("pitch") of the longitudinal grating, i.e. of the grating period. Such a laser can be tuned by controlling the temperature of the laser within a wavelength interval of about 5 nm, since in the typical case the wavelength varies by about 0.1 nm/K and since semiconductor lasers cannot be operated at too high temperatures due to increasing threshold current and a reduced output power of the emitted light for increasing temperature. It means that lasers have to be manufactured in different wavelength classes and that when installing transmitter equipment for wavelength multiplexing correct components have to be selected. It also means that the emitted wavelength cannot be easily changed within a larger wavelength range, i.e. a change to an arbitrary channel cannot easily be made. Possibly only a change of channels can be made for lasers operated at wavelengths which are close to each other. However, such channel changes can be of interest in flexible optical networks comprising optical cross connections (OXC) and optical multiplexers having an add and drop function (OADM, Optical Add/Drop Multi-plexers).
Different proposals have been presented in order to achieve lasers having a wider range in which the wavelength can be selected. These proposals comprise different variants of DBR lasers, in which the reflection maximum of the grating can be displaced by injecting current or by heating the wave guide locally or by subjecting the device to an electrostatic field. One proposal is based on the method that a DFB laser is divided in different segments and the current is varied in the different segments. A third proposal is based on the method that the laser cavity is divided in different subcavities having somewhat different lengths and interference is used between the different cavities in order to define the wavelength which is to be emitted, so called Y-lasers or C.sup.3 -lasers. A problem associated with all these types is that the tuning mechanism is relatively complicated such as that it requires complicated control algorithms and that all those types which are based on current injection in order to change the refractive index, potentially suffer from problems associated with the reliability of the devices.